The invention relates to a process for the elution of fluorinated emulsifiers from anion exchanger resins, to corresponding mixtures for carrying out this process, and to the further work-up of the eluate.
In particular, the invention relates to a process for the elution of fluorinated emulsifiers bound to an anion exchanger resin, in which the resin is brought into contact with a mixture essentially consisting of
a) water,
b) a compound of the formula
Mxe2x80x94X
in which M is an alkali metal or alkylammonium ion, and X is hydroxyl, fluoride or chloride, and
c) at least one organic solvent which completely dissolves the other components a) and b) and thus provides a sufficient quantity of anions Xxe2x88x92for the elution of the emulsifiers from the anion exchanger resin.
The mixture for the elution preferably essentially consists of, in percent by weight,
a) from 15 to 40% of water,
b) from 1 to 10 of the compound Mxe2x80x94X, and
c) from 60 to 70% of the solvent.
A particularly preferred mixture essentially consists of
a) from 18 to 35% of water,
b) from 2 to 8% of Mxe2x80x94X, and
c) from 60 to 70% of solvent.
Preferred solvents, which can be employed individually or as a mixture, are alkanols having 1 to 4 carbon atoms, acetone, mono- and dialkyl ethers of monoglycol and diglycol, where the term alkyl groups here is taken to mean methyl or ethyl. Particularly preferred solvents are methanol, dimethyl monoglycol ether and dimethyl diglycol ether.
Preferred cations M+are lithium, sodium, potassium, tetramethylammonium and tetraethylammonium, and the preferred anion Xxe2x88x92is hydroxyl. Surprisingly, particularly effective aqueous solutions are those of alkali metal hydroxides and alkylammonia.
The fluorinated emulsifiers to be eluted have been known for some time and are employed, in particular, in the polymerization of fluorinated olefins since they do not act as telogens. They are essentially fluorinated alkanecarboxylic and -sulfonic acids, in which the alkyl radical is partially or preferably fully fluorinated and is generally linear or terminally branched. Particular preference is given to perfluorooctanoic acid, referred to below as PFOA, where this abbreviation should hereinafter also be taken to mean the other conventional fluorinated emulsifiers.
The recovery of PFOA using anion exchanger resins has been known for some time and is described, for example, in U.S. Pat. No. 3,882,153 and in EP-B-0 014 431. Particularly advantageous recovery methods for PFOA from waste water are described in WO-A-99/62858 and WO-A-99/62830.
Anion exchanger resins are very effective in removing PFQA from aqueous systems. Highly basic anion exchanger resins, in particular, remove PFOA virtually quantitatively from waste water and similar solutions. In practice, more than 95% of the PFOA present can be recovered in this way. The full capacity of the ion exchanger can be utilized.
However, the strength of the adsorption of the PFOA to the exchanger resin makes elution more difficult. For example, if a loaded, highly basic anion exchanger is eluted with a one-molar, aqueous solution of ammonia, sodium hydroxide or potassium fluoride, PFOA concentrations in the eluate in the order of only 0.1 mmol/1 are obtained. However, work-up of such considerable volumes of waste water is not economical.
On the other hand, weakly basic anion exchanger resins are not as efficient in the recovery of PFOA from the aqueous system. Thus, these resins exhibit premature xe2x80x9cdiffusexe2x80x9d break-through, i.e. the PFOA is released back into the treated aqueous system in small amounts. It is thus not possible to reduce the PFOA reliably to levels below about 5 ppm (corresponding to 7.5 xc2x710xe2x88x926 mol/1) from large volumes of waste water. Such low concentrations are desired for environmental protection reasons since PFOA has poor biodegradability. This applies in particular to waste water containing nonionic emulsifiers, as employed, for example, in concentration by ultrafiltration (EP-B-0 632 009).
In the process disclosed in EP-B-0 014 431, the PFOA is eluted from the anion exchanger using a mixture of a water-soluble solvent, such as methanol, and small amounts of a mineral acid, such as sulfuric acid or hydrochloric acid. In this way, an eluate containing up to 400 mmol/1 of PFOA is obtained using a mixture of 89% by weight of methanol, 4% by weight of sulfuric acid and 7% by weight of water. The eluate usually separates into two layers, the lower layer essentially consisting of PFOA and the upper layer corresponding approximately to the elution mixture. On a larger scale, however, this phase separation does not occur reliably, meaning that recovery of the upper phase for elution is not readily possible (and thus, for example, makes a continuous process more difficult). For regeneration of the ion exchanger resin, about 5 bed volumes of eluate mixture are required, which in practice means from about 5 to 10 m3 of eluate mixture which is a fire risk. The associated equipment complexity is considerable.
By contrast, the above-mentioned mixture, which does not have these disadvantages, is employed for the elution in accordance with the invention. In addition, the process according to the invention allows simple and effective work-up of the eluent, which is firstly subjected to steam distillation until it is essentially free from volatile substances, after which the emulsifier acid in the steam distillation residue is liberated using a sufficiently strong acid. The liberated and thus free (emulsifier) acid is then advantageously distilled off and expediently collected in aqueous ammonia solution since the emulsifier acid is usually employed in the form of the ammonium salt in the polymerization of the fluorinated olefins.
The invention thus relates to an economical process which can be controlled reliably on a large scale for the elution of fluorinated emulsifiers and their recovery in such pure form that they satisfy the requirements made for use in polymerization.